Point-Dependent Resource Symbol Configuration in a Wireless Cell

ABSTRACT

A high-power point and one or more low-power points transmit signals associated with the same cell-identifier in a heterogeneous cell deployment. Coverage areas corresponding to the low-power points fall at least partly within the coverage area for the high-power point, so that mobile stations within range of a low-power point are also within range of the high-power point. Channel-state-information reference symbols, CSI-RS, are transmitted using different CSI-RS resources on different transmission points within the cell, while configuration of CSI-RS measurement resources is conducted on a UE-specific basis. The choice of measurement resources to be used is determined by the network, based on the properties of the channels from the transmission points to the UE. As the UE moves around within the cell, the network tracks the channel properties and reconfigures the CSI-RS resources measured by the UE, to correspond to the resource of the closest transmission point or points.

RELATED APPLICATION

This application claims priority to U.S. Provisional Application Ser.No. 61/440,923, filed 9 Feb. 2011, the entire contents of which areincorporated by reference herein.

TECHNICAL FIELD

The present invention relates generally to the control of devices inwireless communication networks, and more particularly relates totechniques for allocating and using reference signals in networks havingheterogeneous cell deployments.

BACKGROUND

The 3^(rd)-Generation Partnership Project (3GPP) is continuingdevelopment of the fourth-generation wireless network technologies knownas Long-Term Evolution (LTE). Improved support for heterogeneous networkoperations is part of the ongoing specification of 3GPP LTE Release-10,and further improvements are being discussed in the context of newfeatures for Release-11. In heterogeneous networks, a mixture of cellsof different sizes and overlapping coverage areas are deployed.

One example of such a deployment is seen in the system 100 illustratedin FIG. 1, where several pico-cells 120, each with a respective coveragearea 150, are deployed within the larger coverage area 140 of amacro-cell 110. The system 100 of FIG. 1 is suggestive of a wide-areawireless network deployment. However, other examples of low power nodes,also referred to as “points,” in heterogeneous networks are home basestations and relays. Throughout this document, nodes or points in anetwork are often referred to as being of a certain type, e.g., a“macro” node, or a “pico” point. However, unless explicitly statedotherwise, this should not be interpreted as an absolute quantificationof the role of the node or point in the network but rather as aconvenient way of discussing the roles of different nodes or pointsrelative to one another. Thus, a discussion about macro- and pico-cellscould just as well be applicable to the interaction between micro-cellsand femto-cells, for example.

One aim of deploying low-power nodes such as pico base stations withinthe macro coverage area is to improve system capacity, by means ofcell-splitting gains. In addition to improving overall system capacity,this approach also allows users to be provided with a wide-areaexperience of very-high-speed data access, throughout the network.Heterogeneous deployments are in particular effective to cover traffichotspots, i.e., small geographical areas with high user densities. Theseareas can be served by pico cells, for example, as an alternativedeployment to a denser macro network.

The most basic means to operate heterogeneous networks is to applyfrequency separation between the different layers. For instance, themacro-cell 110 and pico-cells 120 pictured in FIG. 1 can be configuredto operate on different, non-overlapping carrier frequencies, thusavoiding any interference between the layers. With no macro-cellinterference towards the under-laid cells, cell-splitting gains areachieved when all resources can simultaneously be used by the under-laidcells.

One drawback of operating layers on different carrier frequencies isthat it may lead to inefficiencies in resource utilization. For example,if there is a low level of activity in the pico-cells, it could be moreefficient to use all carrier frequencies in the macro-cell, and thenbasically switch off the pico-cells. However, the split of carrierfrequencies across layers in this basic configuration is typically donein a static manner.

Another approach to operating a heterogeneous network is to share radioresources between layers. Thus, two (or more) layers can use the samecarrier frequencies, by coordinating transmissions across macro- andunder-laid cells. This type of coordination is referred to as inter-cellinterference coordination (ICIC). With this approach, certain radioresources are allocated to the macro cells for a given time period,whereas the remaining resources can be accessed by the under-laid cellswithout interference from the macro cell. Depending on the trafficsituations across the layers, this resource split can change over timeto accommodate different traffic demands. In contrast to the earlierdescribed static allocation of carrier frequencies, this way of sharingradio resources across layers can be made more or less dynamic dependingon the implementation of the interface between the nodes. In LTE, forexample, an X2 interface has been specified in order to exchangedifferent types of information between base station nodes, forcoordination of resources. One example of such information exchange isthat a base station can inform other base stations that it will reducetransmit power on certain resources.

Time synchronization between base station nodes is generally required toensure that ICIC across layers will work efficiently in heterogeneousnetworks. This is of particular importance for time-domain-based ICICschemes, where resources are shared in time on the same carrier.

Orthogonal Frequency-Division Multiplexing (OFDM) technology is a keyunderlying component of LTE. As is well known to those skilled in theart, OFDM is a digital multi-carrier modulation scheme employing a largenumber of closely-spaced orthogonal sub-carriers. Each sub-carrier isseparately modulated using conventional modulation techniques andchannel coding schemes. In particular, 3GPP has specified OrthogonalFrequency Division Multiple Access (OFDMA) for the downlinktransmissions from the base station to a mobile terminal, and singlecarrier frequency division multiple access (SC-FDMA) for uplinktransmissions from a mobile terminal to a base station. Both multipleaccess schemes permit the available sub-carriers to be allocated amongseveral users.

SC-FDMA technology employs specially formed OFDM signals, and istherefore often called “pre-coded OFDM” or Discrete-Fourier-Transform(DFT)-spread OFDM. Although similar in many respects to conventionalOFDMA technology, SC-FDMA signals offer a reduced peak-to-average powerratio (PAPR) compared to OFDMA signals, thus allowing transmitter poweramplifiers to be operated more efficiently. This in turn facilitatesmore efficient usage of a mobile terminal's limited battery resources.(SC-FDMA is described more fully in Myung, et al., “Single Carrier FDMAfor Uplink Wireless Transmission,” IEEE Vehicular Technology Magazine,vol. 1, no. 3, September 2006, pp. 30-38.)

The basic LTE physical resource can be seen as a time-frequency grid.This concept is illustrated in FIG. 2, which shows a number of so-calledsubcarriers in the frequency domain, at a frequency spacing of Δf,divided into OFDM symbol intervals in the time domain. Each individualelement of the resource grid 210 is called a resource element 220, andcorresponds to one subcarrier during one OFDM symbol interval, on agiven antenna port. One of the unique aspects of OFDM is that eachsymbol 230 begins with a cyclic prefix 240, which is essentially areproduction of the last portion of the symbol 230 affixed to thebeginning. This feature minimizes problems from multipath, over a widerange of radio signal environments.

In the time domain, LTE downlink transmissions are organized into radioframes of ten milliseconds each, each radio frame consisting of tenequally-sized subframes of one millisecond duration. This is illustratedin FIG. 3, where an LTE signal 310 includes several frames 320, each ofwhich is divided into ten subframes 330. Not shown in FIG. 3 is thateach subframe 330 is further divided into two slots, each of which is0.5 milliseconds in duration.LTE link resources are organized into “resource blocks,” defined astime-frequency blocks with a duration of 0.5 milliseconds, correspondingto one slot, and encompassing a bandwidth of 180 kHz, corresponding to12 contiguous sub-carriers with a spacing of 15 kHz. Resource blocks arenumbered in the frequency domain, starting with 0 from one end of thesystem bandwidth. Two time-consecutive resource blocks represent aresource block pair, and correspond to the time interval upon whichscheduling operates. Of course, the exact definition of a resource blockmay vary between LTE and similar systems, and the inventive methods andapparatus described herein are not limited to the numbers used herein.

In general, however, resource blocks may be dynamically assigned tomobile terminals, and may be assigned independently for the uplink andthe downlink. Depending on a mobile terminal's data throughput needs,the system resources allocated to it may be increased by allocatingresource blocks across several sub-frames, or across several frequencyblocks, or both. Thus, the instantaneous bandwidth allocated to a mobileterminal in a scheduling process may be dynamically adapted to respondto changing conditions.

For scheduling of downlink data, the base station transmits controlinformation in each subframe. This control information identifies themobile terminals to which data is targeted and the resource blocks, inthe current downlink subframe, that are carrying the data for eachterminal. The first one, two, three, or four OFDM symbols in eachsubframe are used to carry this control signaling. In FIG. 4, a downlinksubframe 410 is shown, with three OFDM symbols allocated to controlregion 420. The control region 420 consists primarily of control dataelements 434, but also includes a number of reference symbols 432, usedby the receiving station to measure channel conditions. These referencesymbols 432 are interspersed at pre-determined locations throughout thecontrol region 420 and among the data symbols 436 in the data portion430 of the subframe 410.

Transmissions in LTE are dynamically scheduled in each subframe, wherethe base station transmits downlink assignments/uplink grants to certainmobile terminals (user equipment, or UEs, in 3GPP terminology) via thephysical downlink control channel (PDCCH). The PDCCHs are transmitted inthe control region of the OFDM signal, i.e., in the first OFDM symbol(s)of each subframe, and span all or almost all of the entire systembandwidth. A UE that has decoded a downlink assignment, carried by aPDCCH, knows which resource elements in the subframe that contain dataaimed for that particular UE. Similarly, upon receiving an uplink grant,the UE knows which time-frequency resources it should transmit upon. Inthe LTE downlink, data is carried by the physical downlink sharedchannel (PDSCH) and in the uplink the corresponding channel is referredto as the physical uplink shared channel (PUSCH).

LTE also employs multiple modulation formats, including at least QPSK,16-QAM, and 64-QAM, as well as advanced coding techniques, so that datathroughput may be optimized for any of a variety of signal conditions.Depending on the signal conditions and the desired data rate, a suitablecombination of modulation format, coding scheme, and bandwidth ischosen, generally to maximize the system throughput. Power control isalso employed to ensure acceptable bit error rates while minimizinginterference between cells. In addition, LTE uses a hybrid-ARQ (HARQ)error correction protocol where, after receiving downlink data in asubframe, the terminal attempts to decode it and reports to the basestation whether the decoding was successful (ACK) or not (NACK). In theevent of an unsuccessful decoding attempt, the base station canretransmit the erroneous data.

SUMMARY

In several embodiments of the invention, channel-state-informationreference symbols (CSI-RS) are transmitted using different CSI-RSresources on different transmission points within the same cell, whileconfiguration of CSI-RS measurement resources is conducted on aUE-specific basis. The choice of measurement resources to be used isdetermined by the network, based on the properties of the channels fromthe transmission points to the UE of interest. As a UE moves aroundwithin the cell, the network tracks the channel properties andreconfigures the CSI-RS resources measured by the UE, to correspond tothe resource of the “closest” transmission point or points.

More particularly, methods are provided for collectingchannel-state-information (CSI) feedback in a wireless network cell thatfeatures a heterogeneous deployment of transmitting points, i.e.,including a plurality of geographically separated transmission pointssharing a cell identifier and that include a primary transmission point,having a first coverage area, and one or more secondary transmissionpoints, each having a corresponding coverage area that is at leastpartly within the first coverage area. These methods may be carried outat one or nodes in the radio access network, such as at a control nodeconnected via signaling interfaces to each of the transmission points.

In an example method, the network receives CSI feedback from a mobilestation, based on measurements of first CSI reference symbols (CSI-RS)transmitted on a first CSI-RS resource from a first one of thetransmission points. The network then detects that the mobile stationhas approached a second one of the transmission points, differing fromthe first one of the transmission points. In some cases, this detectionis performed by measuring one or more uplink transmissions from themobile station at the second one of the transmission points andassessing channel strength based on the measurements. The measureduplink transmission may include, for example, one or more of a SoundingReference Signal (SRS), a Physical Uplink Control Channel (PUCCH)transmission, and a Physical Uplink Shared Channel (PUSCH) transmission.

The network then reconfigures the mobile station to measure CSI-RS on asecond CSI-RS resource; these CSI-RS are transmitted from the second oneof the transmission points. Finally, the network again receives CSIfeedback from the mobile station, this time based on measurements ofsecond CSI-RS transmitted on the second CSI-RS resource from the secondone of the transmission points.

In some embodiments, such as in a CoMP scenario, the network canconfigure the mobile station to also measure CSI-RS on a third CSI-RSresource transmitted from a third one of the transmission points at thesame general time that CSI-RS are also being transmitted from the secondtransmission point. In this case, the CSI feedback received from themobile station is further based on measurements of the third CSI-RS, andis thus useful for characterizing the compound channel between themobile station and the two (or more) different transmission points.

Another example process for collecting channel-state-information (CSI)feedback begins with the transmitting of CSI-RS from all of severalpoints in the cell, over a given time interval. In many instances,CSI-RS will be transmitted more-or-less simultaneously from all pointsin the cell, i.e., from all points that share the same cell-id, but thisis not strictly necessary.

A subset of the CSI-RS resources are selected for CSI feedback from amobile station, based on channel strength measurements corresponding tothe mobile station and the transmission points. The mobile station isthen configured to provide CSI feedback for at least the selected CSI-RSresources for use in evaluating the channel between the mobile stationand one or several of the transmission points.

In some cases, selecting the subset of the CSI-RS resources is based onmeasurements of an uplink transmission at one or more of thetransmission points. Again, this measured uplink transmission mayinclude one or more of a Sounding Reference Signal (SRS), a PhysicalUplink Control Channel (PUCCH) transmission, and a Physical UplinkShared Channel (PUSCH) transmission. In other cases, the subset of theCSI-RS resources is based on measurement data sent by the mobile stationto one or more of the transmission points. In still other cases, acombination of both sources of information may be used.

Apparatus for carrying out the various processes disclosed herein arealso described, including a system of transmitting nodes in a wirelessnetwork, a corresponding control unit, and a corresponding mobilestation. Of course, the present invention is not limited to the featuresand advantages summarized above. Indeed, those skilled in the art willrecognize additional features and advantages of the present inventionupon reading the following detailed description and viewing the attacheddrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates several pico-cells overlaid by a macro-cell.

FIG. 2 illustrates features of the OFDM time-frequency resource grid.

FIG. 3 illustrates the time-domain structure of an LTE signal.

FIG. 4 illustrates features of an LTE downlink subframe.

FIG. 5 illustrates the mapping of CSI-RS to an LTE resource grid fortwo, four, and eight antenna ports.

FIG. 6 illustrates the differences between uplink and downlink coveragein a mixed cell scenario.

FIG. 7 illustrates the use of inter-cell interference coordination indownlink subframes in a heterogeneous network.

FIG. 8 illustrates a heterogeneous cell deployment where a separatecell-id is used for each point.

FIG. 9 illustrates a heterogeneous cell deployment where the cell-id isshared between the macro-point and pico-points in the macro-point'scoverage area.

FIG. 10 is a process flow diagram illustrating a method for collectingchannel-state-information feedback in a heterogeneous cell deployment.

FIG. 11 is a process flow diagram illustrating another method forcollecting channel-state-information feedback in a heterogeneous celldeployment.

FIG. 12 is a block diagram illustrating features of nodes in aheterogeneous cell deployment.

FIG. 13 is a process flow diagram illustrating a method for providingchannel-state-information in a heterogeneous cell deployment.

FIG. 14 is a block diagram illustrating components of an example mobilestation.

DETAILED DESCRIPTION

Various embodiments of the present invention are now described withreference to the drawings, where like reference numerals are used torefer to like elements throughout. In the following description,numerous specific details are set forth for purposes of explanation, inorder to provide a thorough understanding of one or more embodiments. Itwill be evident to one of ordinary skill in the art, however, that someembodiments of the present invention may be implemented or practicedwithout one or more of these specific details. In other instances,well-known structures and devices are shown in block diagram form inorder to facilitate describing embodiments.

Note that although terminology from 3GPP's specifications for LTE andLTE-Advanced is used throughout this document to exemplify theinvention, this should not be seen as limiting the scope of theinvention to only these systems. Other wireless systems including oradapted to include heterogeneous cell deployments may also benefit fromexploiting the ideas covered herein.

Demodulation of transmitted data generally requires estimation of theradio channel. In LTE systems, this is done using transmitted referencesymbols (RS), i.e., transmitted symbols having values that are alreadyknown to the receiver. In LTE, cell-specific reference symbols (CRS) aretransmitted in all downlink subframes. In addition to assisting downlinkchannel estimation, the CRS are also used for mobility measurementsperformed by the UEs.

The CRS are generally intended for use by all the mobile terminals inthe coverage area. To support improved channel estimation, especiallywhen multiple-input multiple-output (MIMO) transmission techniques areused, LTE also supports UE-specific RS, which are targeted to individualmobile terminals and are intended specifically for channel estimationfor demodulation purposes.

FIG. 4 illustrates how the mapping of physical control/data channels andsignals can be done on resource elements within a downlink subframe 410.In the pictured example, the PDCCHs occupy only the first out of thethree possible OFDM symbols that make up the control region 420, so inthis particular case the mapping of data can begin at the second OFDMsymbol. Since the CRS are common to all UEs in the cell, thetransmission of CRS cannot be easily adapted to suit the needs of aparticular UE. This is in contrast to UE-specific RS, by means of whicheach UE can have RS of its own placed in the data region 430 of FIG. 4,as part of PDSCH.

The length (one, two, or three symbols) of the control region that isused to carry PDCCH can vary on a subframe-to-subframe basis, and issignaled to the UE in the Physical Control Format Indicator CHannel(PCFICH). The PCFICH is transmitted within the control region, atlocations known by terminals. Once a terminal has decoded the PCFICH, itthen knows the size of the control region and in which OFDM symbol thedata transmission starts. Also transmitted in the control region is thePhysical Hybrid-ARQ Indicator Channel. This channel carries ACK/NACKresponses to a terminal, to inform the mobile terminal whether theuplink data transmission in a previous subframe was successfully decodedby the base station.

As noted above, CRS are not the only reference symbols available in LTE.As of LTE Release-10, a new RS concept was introduced. SeparateUE-specific RS for demodulation of PDSCH are supported in Release 10, asare RS specifically provided for measuring the channel for the purposeof generating channel state information (CSI) feedback from the UE. Thelatter reference symbols are referred to as CSI-RS. CSI-RS are nottransmitted in every subframe, and they are generally sparser in timeand frequency than RS used for demodulation. CSI-RS transmissions maytake place every fifth, tenth, twentieth, fortieth, or eightiethsubframe, as determined by a periodicity parameter and a subframeoffset, each of which are configured by Radio Resource Control (RRC)signaling.

A UE operating in connected mode can be requested by the base station toperform channel state information (CSI) reporting. This reporting caninclude, for example, reporting a suitable rank indicator (RI) and oneor more precoding matrix indices (PMIs), given the observed channelconditions, as well as a channel quality indicator (CQI). Other types ofCSI are also conceivable, including explicit channel feedback andinterference covariance feedback. The CSI feedback assists the basestation in scheduling, including deciding which subframe and resourceblocks to use for the transmission, as well as deciding whichtransmission scheme and/or precoder should be used. The CSI feedbackalso provides information that can be used to determine a proper userbit-rate for the transmission, i.e., for link adaptation.

In LTE, both periodic and aperiodic CSI reporting are supported. In thecase of periodic CSI reporting, the terminal reports the CSImeasurements on a configured periodic time basis, using the physicaluplink control channel (PUCCH). With aperiodic reporting, the CSIfeedback is transmitted on the physical uplink shared channel (PUSCH) atpre-specified time instants after receiving the CSI grant from the basestation. With aperiodic CSI reports, the base station can thus requestCSI that reflects downlink radio conditions in a particular subframe.

A detailed illustration of which resource elements within a resourceblock pair that may potentially be occupied by the new UE-specific RSand CSI-RS is provided in FIG. 5, for the cases in which two, four, andeight transmitter antenna ports are used for the CSI transmission. TheCSI-RS utilizes an orthogonal cover code of length two to overlay twoantenna ports on two consecutive resource elements. In other words, theCSI-RS are allocated in pairs, where two orthogonal codes of length twoare transmitted simultaneously, using the same pair of allocatedresource elements, from a pair of antenna ports at the base station.

In FIG. 5, the CSI-RS resource elements are designated with numbers,which correspond to antenna port numbers. In the left-hand diagram,corresponding to the case of two CSI-RS antenna ports, the possiblepositions for the CSI-RS are labeled “0” and “1”, corresponding toantenna ports 0 and 1.

As can be seen in FIG. 5, many different CSI-RS pattern are available.For the case of two CSI-RS antenna ports, for instance, where eachCSI-RS pair can be separately configured, there are twenty differentpatterns within a subframe. When there are four CSI-RS antenna ports,the CSI-RS pairs are assigned two at a time; thus the number of possiblepatterns is ten. For the case of eight CSI-RS antenna ports, fivepatterns are available. For TDD mode, some additional CSI-RS patternsare available.

In the following discussion, the term “CSI-RS resource” is used. ACSI-RS resource corresponds to a particular pattern present in aparticular subframe. Thus two different patterns in the same subframeconstitute two distinct CSI-RSI resources. Likewise, the application ofthe same CSI-RS pattern to two different subframes again represents twoseparate instances of a CSI-RS resource, and the two instances are thusagain to be considered distinct CSI-RS resources.

Any of the various CSI-RS patterns pictured in FIG. 5 may alsocorrespond to so-called zero-power CSI-RS, which are also referred to asmuted REs. A zero-power CSI-RS is a CSI-RS pattern whose resourceelements are silent, i.e., there is no transmitted signal on thoseresource elements. These silent patterns are configured with aresolution corresponding to the four-antenna-port CSI-RS patterns.Hence, the smallest unit of silence that may be configured correspondsto four REs.

The purpose of zero-power CSI-RS is to raise thesignal-to-interference-plus-noise ratio (SINR) for CSI-RS in a givencell, by configuring zero-power CSI-RS in interfering cells so that theresource elements that would otherwise cause interference are silent.Thus, a CSI-RS pattern in a given cell is matched with a correspondingzero-power CSI-RS pattern in interfering cells.

Raising the SINR level for CSI-RS measurements is particularly importantin applications such as coordinated multi point (CoMP) or inheterogeneous deployments. In CoMP, the UE is likely to need to measurethe channel from non-serving cells. Interference from the much strongerserving cell would make those measurements difficult, if not impossible.Zero-power CSI-RS are also needed in heterogeneous deployments, wherezero-power CSI-RS in the macro-layer are configured to coincide withCSI-RS transmissions in the pico-layer. This avoids strong interferencefrom macro nodes when UEs measure the channel to a pico-node.

The PDSCH, which carries data targeted for mobile stations, is mappedaround the resource elements occupied by CSI-RS and zero-power CSI-RS,so it is important that both the network and the UE are assuming thesame CSI-RS and zero power CSI-RS configurations. Otherwise, the UE maybe unable to properly decode the PDSCH in subframes that contain CSI-RSor their zero-power counterparts.

The CSI-RS discussed above is used for measurements of the downlinkchannel, i.e., from a base station to a mobile terminal. In the uplink,so-called sounding reference symbols (SRS) may be used for acquiring CSIabout the uplink channel from the UE to a receiving node. When SRS isused, it is transmitted on the last DFT-spread OFDM symbol of asubframe. SRS can be configured for periodic transmission as well fordynamic triggering as part of the uplink grant. The primary use for SRSis to aid the scheduling and link adaptation in the uplink. Fortime-division duplex (TDD) LTE systems, however, SRS is sometimes usedto determine beam-forming weights for the downlink, by exploiting thefact that the downlink and uplink channels are the same when the samecarrier frequency is used for downlink and uplink (channel reciprocity).

While PUSCH carries data in the uplink, PUCCH is used for control. PUCCHis a narrowband channel using a resource block pair where the tworesource blocks are on opposite sides of the potential schedulingbandwidth. PUCCH is used for conveying ACK/NACKs, periodic CSI feedback,and scheduling request to the network.

Before an LTE terminal can communicate with an LTE network it first hasto find and acquire synchronization to a cell within the network, aprocess known as cell search. Next, the UE has to receive and decodesystem information needed to communicate with and operate properlywithin the cell. Finally, the UE can access the cell by means of theso-called random-access procedure.

In order to support mobility, a terminal needs to continuously searchfor, synchronize to, and estimate the reception quality of both itsserving cell and neighbor cells. The reception quality of the neighborcells, in relation to the reception quality of the current cell, is thenevaluated in order to determine whether a handover (for terminals inconnected mode) or cell re-selection (for terminals in idle mode) shouldbe carried out. For terminals in connected mode, the handover decisionis taken by the network, based on measurement reports provided by theterminals. Examples of such reports are reference signal received power(RSRP) and reference signal received quality (RSRQ).

The results of these measurements, which are possibly complemented by aconfigurable offset, can be used in several ways. The UE can, forexample, be connected to the cell with the strongest received power.Alternatively, the UE can be assigned to the cell with the best pathgain. An approach somewhere between these alternatives may be used.

These selection strategies do not always result in the same selectedcell for any given set of circumstances, since the base station outputpowers of cells of different type are different. This is sometimesreferred to as link imbalance. For example, the output power of a picobase station or a relay node is often on the order of 30 dBm (1 watt) orless, while a macro base station can have an output power of 46 dBm (40watts). Consequently, even in the proximity of the pico cell, thedownlink signal strength from the macro cell can be larger than that ofthe pico cell. From a downlink perspective, it is often better to selecta cell based on downlink received power, whereas from an uplinkperspective, it would be better to select a cell based on the path loss.

These alternative cell selection approaches are illustrated in FIG. 6.The solid lines emanating from each of macro-cell 110 and pico-cell 120represent the received power at each point between the two cells. Theselines intersect, i.e., are equal, at border 540. Accordingly, a UEwithin region 510 will see a stronger received signal from the pico-cell120, and will get the best downlink performance if it selects pico-cell120. The dashed lines issuing from pico-cell 120 and macro-cell 110, onthe other hand, represent the path loss between a UE at a given pointand either the macro-cell 110 or the pico-cell 120. Because the pathloss is not weighted by the transmitter output power, these linesintersect at a point halfway between macro-cell 110 and pico-cell 120,as seen at the boundary 530. A UE outside region 520, then, willexperience a lower path loss to macro-cell 110 than to pico-cell 120,and will thus achieve better uplink performance if it selects macro-cell110. Because of this unbalanced situation, there is a region, i.e., theportion of coverage area 520 that is outside coverage area 510, in whichneither cell is optimal for both downlink and uplink performance at thesame time.

From a system perspective, it might often be better, in the abovescenario, for a given UE to connect to the pico-cell 120 even under somecircumstances where the downlink from macro-cell 110 is much strongerthan the pico cell downlink. However, ICIC across layers will be neededwhen the terminal operates within the region between the uplink anddownlink borders, i.e., the link imbalance zone, as depicted in FIG. 6.Interference coordination across the cell layers is especially importantfor the downlink control signaling. If the interference is not handledappropriately, a terminal that is in the region between the downlink anduplink borders in FIG. 6 and is connected to pico-cell 120 may be unableto receive the downlink control signaling from the pico-cell 120.

One approach to providing ICIC across layers is illustrated in FIG. 7.An interfering macro-cell, which could create downlink interferencetowards a pico-cell, transmits a series of subframes 710, but avoidsscheduling unicast traffic in certain subframes 712. In other words,neither PDCCHs nor PDSCH are transmitted in those subframes 712. In thisway, it is possible to create low-interference subframes, which can beused to protect users of the pico-cell who are operating in the linkimbalance zone.

To carry out this approach, the macro-base station (MeNB) indicates tothe pico-base station (PeNB), via the backhaul interface X2, whichsubframes will not be used to schedule users. The PeNB can then takethis information into account when scheduling users operating within thelink imbalance zone, such that these users are scheduled only insubframes 722 aligned with the low-interference subframes transmitted inthe macro layer. In other words, these users are scheduled only ininterference-protected subframes. Pico-cell users operating within thedownlink border, e.g., within coverage area 510 in FIG. 6, can bescheduled in all subframes, i.e., in both the protected subframes 722 aswell as the remaining, un-protected, subframes in the series ofsubframes 720.

In principle, data transmission (but not control signaling) in differentlayers could also be separated in the frequency domain by ensuring thatscheduling decisions in the two cell layers are non-overlapping in thefrequency domain. This could be facilitated by exchanging coordinationmessages between the different base stations. However, this is notpossible for the control signaling, since the control signaling spansthe full bandwidth of the signal, according to the LTE specifications,and hence a time-domain approach must be used.

The classical way of deploying a network is for each differenttransmission/reception point to provide coverage for a cell that isdistinct from all others. That is, the signals transmitted from orreceived at a point is associated with a cell identifier (cell-id) thatis different from the cell-id employed for other nearby points.Typically, each of these points transmits its own unique signals forbroadcast (PBCH) and sync channels (PSS, SSS).

The concept of a “point” is heavily used in conjunction with techniquesfor coordinated multipoint (CoMP). In this context, a point correspondsto a set of antennas covering essentially the same geographical area ina similar manner. Thus, a point might correspond to one of the sectorsat a site, but it may also correspond to a site having one or moreantennas all intending to cover a similar geographical area. Often,different points represent different sites. Antennas correspond todifferent points when they are sufficiently geographically separatedand/or have antenna diagrams pointing in sufficiently differentdirections. Techniques for CoMP entail introducing dependencies in thescheduling or transmission/reception among different points, in contrastto conventional cellular systems where a point is operated more or lessindependently from the other points, from a scheduling point of view.

The classical strategy of one cell-id per point is depicted in FIG. 8for a heterogeneous deployment where a number of low-power (pico) points120 are placed within the coverage area of a higher power macro point110. In this deployment, the pico-nodes transmit different cellidentifiers, i.e., “cell-id 2”, “cell-id 3”, and “cell-id 4”, from thecell identifier “cell-id 1” transmitted by the macro-cell 110. Note thatsimilar principles obviously also apply to classical macro-cellulardeployments where all points have similar output power and perhaps areplaced in a more regular fashion than what is the case for aheterogeneous deployment.

An alternative to the classical deployment strategy is to instead letall the UEs within a geographical area outlined by the coverage of thehigh power macro point be served with signals associated with the samecell-id. In other words, from a UE perspective, the received signalsappear as though they come from a single cell. This is illustrated inFIG. 9. Here, all of the pico-nodes 120 transmit the same cellidentifier, “cell-id 1”, which is also used by the overlaying macro-cell110.

Note that in both FIGS. 8 and 9 only one macro point is shown; othermacro points would typically use different cell-ids (corresponding todifferent cells) unless they are co-located at the same site(corresponding to other sectors of the macro site). In the latter caseof several co-located macro points, the same cell-id may be sharedacross the co-located macro-points and those pico points that correspondto the union of the coverage areas of the macro points. Sync, BCH andcontrol channels are all transmitted from the high-power point whiledata can be transmitted to a UE also from low-power points by usingshared data transmissions (PDSCH) that rely on UE-specific RS.

Such an approach has benefits for those UEs that are capable ofreceiving PDSCH based on UE-specific RS, while UEs that only support CRSfor PDSCH have to settle for using only the transmission from thehigh-power point, and thus will not benefit in the downlink from thedeployment of extra low-power points. This latter group is likely toinclude at least all Release 8 and 9 UEs for use in FDD LTE systems.

The single cell-id approach for heterogeneous and/or hierarchical celldeployments is geared towards situations in which there is fast backhaulcommunication between the points associated with the same cellidentifier. A typical case would be a base station serving one or moresectors on a macro level as well as having fast fiber connections toremote radio units (RRUs) performing the role of the other points thatshare the same cell-id. Those RRUs could represent low-power points withone or more antennas each. Another example is when all the points have asimilar power class, with no single point having more significance thanthe others. The base station would then handle the signals from all RRUsin a similar manner.

A clear advantage of the shared-cell approach compared with theclassical one is that the handover procedure between cells only needs tobe invoked on a macro basis. Another important advantage is thatinterference from CRS can be greatly reduced, since CRS does not have tobe transmitted from every point. There is also much greater flexibilityin coordination and scheduling among the points, which means the networkcan avoid relying on the inflexible concept of semi-staticallyconfigured low-interference subframes, as illustrated in FIG. 7. Ashared-cell approach also allows decoupling of the downlink from theuplink, so that, for example, path-loss-based reception-point selectioncan be performed for the uplink, without creating a severe interferenceproblem for the downlink, where the UE may be served by a transmissionpoint different from the point used in the uplink receptions.

The shared cell-id approach presents some problems when it comes to CSIfeedback, however. A single cell may now encompass a large number ofantennas, many more than the one to eight transmit antennas for whichLTE procedures for CSI feedback were designed. Also, the overhead due totransmitting CSI-RS tends to become large when many antennas are used bythe cell.

Furthermore, even in cases when there are eight or fewer antennassharing the same cell, the distributed placement of these antennascreates a compound channel to the UE, this compound channel havingproperties that are ill-matched to the design assumptions used fornormal CSI feedback procedures, which are designed to match the channelcharacteristics that result when the antennas are confined to a singletransmission point.

To address these issues, then, in several embodiments of the inventionCSI-RS are transmitted using different CSI-RS resources on differenttransmission points within the same cell, while configuration of CSI-RSmeasurement resources is conducted on a UE-specific basis, where thechoice of measurement resource/resources is/are determined by thenetwork, based on the properties of the channels from the transmissionpoints to the UE of interest. As a UE moves around within the cell, thenetwork tracks the channel properties and reconfigures the CSI-RSresources measured by the UE, to correspond to the resource of the“closest” transmission point or points.

At a high-level, a system configured according to some embodiments ofthe invention includes the following features. First, a set of pointsare associated with the same cell, in that signals emitted from any ofthe points in the set are associated with the same cell-id. For example,a given cell in the system might include two or more low-power points,in addition to the high-power point. As discussed earlier, the coverageareas of the low-power points may overlap or fall completely within thecoverage area of the high-power point. A control node in the networkconfigures a given UE operating within the cell to measure the channelproperties based on a CSI-RS resource transmitted from one of the pointswhen the UE is sufficiently close to that point. Here, “close” is meantin a radio sense, in that a UE that is close to a transmitting pointreceives the transmitted signal well. Of course, “close” in the radiosense will often, but not always, coincide with “close” in thegeographical sense.

When the UE moves closer to another point, however, the networkconfigures the UE to instead measure the channel properties based onCSI-RS transmitted from that other point, using a different CSI-RSresource. The choice of which CSI-RS resource (or resources) that the UEshould use at any given time may be made by the network based onmeasurements on uplink signals (e.g. SRS, PUSCH, PUCCH, etc.), or basedon CSI feedback from the UE (or other UEs), or some combination of both.

In some cases, such as in a coordinated multi-point (CoMP) scenario, itmay be desirable for the UE to measure CSI-RS resources corresponding tomultiple points. In this case, the same general procedure is used,except that references to a single “point” in the discussion immediatelyabove may each be replaced by “a set of points.”

In more detail, for CSI feedback modes that utilize CSI-RS, the UE canbe configured by means of higher layer signaling from the network todetermine which CSI-RS resource to measure on. In various embodiments ofthe present invention, this configuration is UE-specific. Normally, theconfiguration of CSI-RS is performed in a cell-specific manner, so thatall UEs served by the same cell acquire the same configuration and allUEs make measurements using the same CSI-RS resource. In the case ofshared cell-id, however, the UE measurements for CSI feedback need to becarefully controlled from the network to solve the CSI problem.Efficient network control is achieved by configuring the CSI-RS in aUE-specific manner that depends on which transmission point or pointswithin the cell contribute significantly to the received signal for agiven UE.

For example, each transmission point may transmit using a CSI-RSresource (as given by the CSI-RS pattern within a subframe, theperiodicity and the subframe offset) of its own. As the UE approaches aparticular transmission point, the relative strengths of the channelsfrom the different transmission points to the UE are assessed. Based onthis assessment, the network decides when to reconfigure the UE tomeasure CSI-RS on the particular CSI-RS resources that a particulartransmission point is using. The network may acquire channel strengthsfrom measurements of uplink signals, including SRS, PUCCH, PUSCH or frommulti-CSI-RS resource CSI feedback, if such feedback would be supportedin LTE.

Thus, the CSI-RS resource to measure on is configured by the network ina UE-specific manner within the cell, such that the chosen resource islargely determined based on which of the transmission points are bestheard by each UE. As a UE moves between the transmission points, thenetwork tracks the channel properties and reconfigures the CSI-RSresource for the UE to correspond to the resource of the “closest”transmission point.

This CSI-RS reconfiguration procedure is also applicable to cases whenCoMP is employed. To support effective coordination among the points,the UE needs to feed back CSI corresponding to the channel formedbetween the UE and multiple transmission points. As an example, a UE maybe configured so that it feeds back CSI corresponding to the two orthree strongest channels or transmission points. Instead of configuringonly one CSI-RS resource for the UE of interest, the network now needsto configure multiple CSI-RS measurement resources within the cell. Thenetwork needs to monitor the radio conditions to the points relevant forthe UE, and as the radio conditions for the UE varies, the network wouldreconfigure one or more of the resources with the goal that the UEmeasures on relevant points (i.e., points which the UE hearssufficiently well). Just as for the non-CoMP CSI-RS case, measurementson uplink signals and their strengths at different reception pointscould be used as decision basis for the CSI-RS resource measurement set.

Alternatively, a UE may be configured to measure on a larger set ofCSI-RS resources, after which measurements a subset of those CSI-RSresources is chosen for the actual CSI feedback. Thus, the best CSI-RSresource measurement subset is determined by actual measurements of thelarger set. This measurement on the larger set is, of course, performedby the UE. However, the selection of the best CSI-RS measurement set forevaluating the channel conditions can be performed either by the UE orby the network. In the latter case, the UE sends the measurementscorresponding to the larger CSI-RS resource set to the network and thenthe network instructs the UE which CSI-RS resources to measure on. Inthe former case, the UE needs to send only CSI for the smaller subset ofresources.

Use of the techniques disclosed herein can help ensure that efficientCSI feedback matching the channel properties is obtained from the UEs.Without these transmission and configuration strategies, the UEs couldfeed back CSI that is ill-matched to the dominating parts of the channelfrom the transmission points, resulting in a loss of array gain anddifficulties in performing multi-user MIMO scheduling.

Given the above details of UE-specific configuration of CSI-RSresources, it will be appreciated that FIGS. 10 and 11 illustrategeneralized procedures for collecting channel-state-information (CSI)feedback in a wireless network cell that features a heterogeneousdeployment of transmitting points, i.e., including a plurality ofgeographically separated transmission points sharing a cell identifierand that include a primary transmission point, having a first coveragearea, and one or more secondary transmission points, each having acorresponding coverage area that is at least partly within the firstcoverage area. The illustrated method is carried out at one or nodes inthe radio access network, such as at a control node connected viasignaling interfaces to each of the transmission points.

As shown at block 1010 of FIG. 10, the network receives CSI feedbackfrom a mobile station, based on measurements of first CSI referencesymbols (CSI-RS) transmitted on a first CSI-RS resource from a first oneof the transmission points. The network then detects that the mobilestation has approached a second one of the transmission points,differing from the first one of the transmission points. This is shownat block 1020. While several techniques for detecting that a mobilestation is approaching a particular transmission point are possible, theterm “approaching” here is generally intended to mean that the mobilestation is approaching in a radio signal sense, in that the radiochannel between the mobile station and the transmission point isimproving. Thus, in some cases, this detection may be performed bymeasuring one or more uplink transmissions from the mobile station atthe second one of the transmission points and assessing channel strengthbased on the measurements. The measured uplink transmission may includeone or more of a Sounding Reference Signal (SRS), a Physical UplinkControl Channel (PUCCH) transmission, and a Physical Uplink SharedChannel (PUSCH) transmission.

As shown at block 1030, the network then reconfigures the mobile stationto measure CSI-RS on a second CSI-RS resource; these CSI-RS aretransmitted from the second one of the transmission points, as shown atblock 1040. Finally, as shown at block 1050, the network again receivesCSI feedback from the mobile station, this time based on measurements ofsecond CSI-RS transmitted on the second CSI-RS resource from the secondone of the transmission points.

The process illustrated in FIG. 10 is not limited to measurements ofCSI-RS transmitted by just a single transmission point at any giventime. In some embodiments, such as in a CoMP scenario, the network canconfigure the mobile station to also measure CSI-RS on a third CSI-RSresource transmitted from a third one of the transmission points at thesame general time that CSI-RS are also being transmitted from the secondtransmission point. In this case, the CSI feedback received from themobile station is further based on measurements of the third CSI-RS, andis thus useful for characterizing the compound channel between themobile station and the two (or more) different transmission points.

FIG. 11 illustrates a closely related process for collectingchannel-state-information (CSI) feedback in a wireless network cellsimilar to those discussed above. This process begins, as shown at block1110, with the transmitting of CSI-RS from all of several points in thecell, over a given time interval. In many instances, CSI-RS will betransmitted more-or-less simultaneously from all points in the cell,i.e., from all points that share the same cell-id, but this is notstrictly necessary.

As shown at block 1120, a subset of the CSI-RS resources are selectedfor CSI feedback from a mobile station, based on channel strengthmeasurements corresponding to the mobile station and the transmissionpoints. The mobile station is then configured to provide CSI feedbackfor at least the selected CSI-RS resources, as shown at block 1130, foruse in evaluating the channel between the mobile station and one orseveral of the transmission points.

In some cases, selecting the subset of the CSI-RS resources is based onmeasurements of an uplink transmission at one or more of thetransmission points. Again, this measured uplink transmission mayinclude one or more of a Sounding Reference Signal (SRS), a PhysicalUplink Control Channel (PUCCH) transmission, and a Physical UplinkShared Channel (PUSCH) transmission. In other cases, the subset of theCSI-RS resources is based on measurement data sent by the mobile stationto one or more of the transmission points. In still other cases, acombination of both sources of information may be used.

Other embodiments of the inventive techniques disclosed herein include awireless system, including a primary node and one or more secondarynodes, corresponding to the methods and techniques described above. Insome cases, the methods/techniques described above will be implementedin a system of transmitting nodes such as the one pictured in detail inFIG. 12.

The system pictured in FIG. 12 includes a macro node 110, two pico nodes120, a UE 130, and an O&M node 190. The macro node 110 is configured tocommunicate with pico nodes 120 and O&M node 190 via inter-base-stationinterface 1204, which comprises suitable network interface hardwarecontrolled by software carrying out network interfacing protocols. Macronode 110 includes a receiver 1202 and transmitter 1206 for communicatingwith UE 130; in some cases receiver 1202 may also be configured tomonitor and/or measure signals transmitted by pico node 120. Receivercircuit 1202 and transmitter circuit 1206 use known radio processing andsignal processing components and techniques, typically according to aparticular telecommunications standard such as the 3GPP standard forLTE-Advanced. Because the various details and engineering tradeoffsassociated with the design of interface circuitry and radio transceivercircuits are well known and are unnecessary to a full understanding ofthe invention, additional details are not shown here.

Macro node 110 further includes a processing circuit 1210, whichincludes one or more microprocessors or microcontrollers, as well asother digital hardware, which may include digital signal processors(DSPs), special-purpose digital logic, and the like. Either or both ofthe microprocessor(s) and the digital hardware may be configured toexecute program code stored in memory 1220, along with stored radioparameters. Again, because the various details and engineering tradeoffsassociated with the design of baseband processing circuitry for mobiledevices and wireless base stations are well known and are unnecessary toa full understanding of the invention, additional details are not shownhere. However, several functional aspects of the processing circuit 1210are shown, including a measuring unit 1212, a control unit 1214, and aconfiguration unit 1216. Configuration unit 216 controls radiotransmitter 1206 to transmit CRS, CSI-RS, PDSCH, etc., under the controlof control unit 1214, which also manages the communications with othernodes via inter-BS interface circuit 1204. Control unit 1214 alsoevaluates data obtained from measuring unit 1212, such as channel stateinformation and/or load information, and controls inter-base-stationcommunication and transmitter configuration accordingly.

Program code stored in memory circuit 1220, which may comprise one orseveral types of memory such as read-only memory (ROM), random-accessmemory, cache memory, flash memory devices, optical storage devices,etc., includes program instructions for executing one or moretelecommunications and/or data communications protocols, as well asinstructions for carrying out all or part of one or more of thetechniques described above, in several embodiments. Radio parametersstored in memory 1220 may include one or more pre-determined tables orother data for supporting these techniques, in some embodiments.

Pico nodes 120 may comprise components and functional blocks verysimilar to those illustrated in macro node 110, with the correspondingcontrol units being responsible for receiving control instructions froma macro node 110 (or other pico node 120) and configuring the piconode's transmitter circuits accordingly.

In some embodiments a macro node 110 may act as a control node for thepurposes of the techniques described above, in that the macro node 110carries out all or part of one of the methods illustrated in FIGS. 10and 11, or variants thereof. In other scenarios, a pico node 120 may actas a similar control node. In still other embodiments, the controlfunctionality may be split among two or more physical nodes, which acttogether as a control node to carry out the above-described techniquesfor collecting channel-state information in a heterogeneous celldeployment. Thus, the term “control node,” as used herein, is notlimited to a unitary piece of equipment at a single physical location,but may also refer to a collection of network equipment operatingtogether.

Still other embodiments of the inventive techniques described hereininclude methods implemented at a mobile station operating in aheterogeneous cell deployment as described above. FIG. 13, inparticular, illustrates an example of one such process for providingchannel-state-information (CSI) feedback in a wireless network cell thatcomprises a plurality of geographically separated transmission pointssharing a cell identifier, the transmission points including a primarytransmission point, having a first coverage area, and one or moresecondary transmission points, each having a corresponding coverage areathat is entirely or substantially within the first coverage area.

The illustrated method begins, as shown at block 1310, with themeasuring, in a mobile station, of signals transmitted from two or moreof the transmission points in the cell. The resulting measurements aresent to at least one of the transmission points, as shown at block 1320.The mobile station then receives configuration information from at leastone of the transmission points, the configuration informationinstructing the mobile station to measure CSI reference symbols (CSI-RS)from two or more CSI-RS resources corresponding to at least two of thetransmission points. This is shown at block 1330. The mobile stationmeasures CSI-RS from the two or more CSI-RS resources, as shown at block1340, and sends CSI feedback to at least one of the transmission points,based on the measured CSI-RS, as shown at block 1350.

An example of a mobile station configured to carry out the method ofFIG. 13, or variants thereof, is shown in FIG. 14. Of course, not everydetail of the mobile station design is shown, but instead a few of thecomponents relevant to the present techniques are pictured. The picturedapparatus includes radio circuitry 1410 and baseband & controlprocessing circuit 1420. Radio circuitry 1410 includes receiver circuitsand transmitter circuits that use known radio processing and signalprocessing components and techniques, typically according to aparticular telecommunications standard such as the 3GPP standard for LTEand/or LTE-Advanced. Again, because the various details and engineeringtradeoffs associated with the design and implementation of suchcircuitry are well known and are unnecessary to a full understanding ofthe invention, additional details are not shown here.

Baseband & control processing circuit 1420 includes one or moremicroprocessors or microcontrollers 1430, as well as other digitalhardware 1435, which may include digital signal processors (DSPs),special-purpose digital logic, and the like. Either or both ofmicroprocessor(s) 1430 and digital hardware 1435 may be configured toexecute program code 1445, which is stored in memory 1440 along withradio parameters 1450. Once more, because the various details andengineering tradeoffs associated with the design of baseband processingcircuitry for mobile devices are well known, additional details are notshown here

The program code 1445 stored in memory circuit 1440, which may compriseone or several types of memory such as read-only memory (ROM),random-access memory, cache memory, flash memory devices, opticalstorage devices, etc., includes program instructions for executing oneor more telecommunications and/or data communications protocols, as wellas instructions for carrying out one or more of the techniques describedabove, in several embodiments. Radio parameters 1450 include variousconfiguration parameters as well as parameters determined from systemmeasurements, such as channel measurements, and may includeconfiguration data indicating which CSI-RS should be measured, as wellas measurement data resulting from such measurements.

Accordingly, in various embodiments of the invention, a processingcircuit, such as the baseband & control processing circuit 1420 of FIG.14, is configured to carry out one or more of the techniques describedabove for providing channel-state-information (CSI) feedback in awireless network cell that comprises a plurality of geographicallyseparated transmission points sharing a cell identifier. As describedabove, this processing circuit is configured with appropriate programcode, stored in one or more suitable memory devices, to implement one ormore of the techniques described herein. Of course, it will beappreciated that not all of the steps of these techniques arenecessarily performed in a single microprocessor or even in a singlemodule.

Examples of several embodiments of the present invention have beendescribed in detail above, with reference to the attached illustrationsof specific embodiments. Because it is not possible, of course, todescribe every conceivable combination of components or techniques,those skilled in the art will appreciate that the present invention canbe implemented in other ways than those specifically set forth herein,without departing from essential characteristics of the invention. Thepresent embodiments are thus to be considered in all respects asillustrative and not restrictive.

1. A method for collecting channel-state-information, CSI, feedback in a wireless network that comprises a plurality of geographically separated transmission points sharing a cell identifier, the method comprising: receiving CSI feedback from a mobile station, based on measurements of first CSI reference symbols, CSI-RS, transmitted on a first CSI-RS resource from a first one of the transmission points; detecting that the mobile station has approached a second one of the transmission points, differing from the first one of the transmission points; reconfiguring the mobile station to measure CSI-RS on a second CSI-RS resource transmitted from the second one of the transmission points; and receiving CSI feedback from the mobile station, based on measurements of second CSI-RS transmitted on the second CSI-RS resource from the second one of the transmission points.
 2. The method of claim 1, wherein the transmission points including a primary, high-power transmission point, having a first coverage area, and one or more secondary, low-power, transmission points, each having a coverage area that is within or substantially within the first coverage area.
 3. The method of claim 1, wherein detecting that the mobile station has approached a second one of the transmission points comprises measuring an uplink transmission from the mobile station at the second one of the transmission points and assessing channel strength based on said measuring.
 4. The method of claim 3, wherein the measured uplink transmission comprises at least one of a Sounding Reference Signal, SRS, a Physical Uplink Control Channel, PUCCH, transmission, and a Physical Uplink Shared Channel, PUSCH, transmission.
 5. The method of claim 1, further comprising: configuring the mobile station to also measure CSI-RS on a third CSI-RS resource transmitted from a third one of the transmission points; and transmitting third CSI reference symbols, CSI-RS, on the third CSI-RS resource, from the third one of the transmission points, simultaneously with said transmitting of second CSI-RS from the second one of the transmission points; wherein the CSI feedback received from the mobile station is further based on measurements of the third CSI-RS.
 6. The method of claim 1, further comprising: transmitting CSI-RS from each of the transmission points, using at least one distinct CSI-RS resource for each of the transmission points; selecting a subset of the CSI-RS resources for CSI feedback from a mobile station, based on channel strength measurements corresponding to the mobile station and the transmission points; and configuring the mobile station to provide CSI feedback for the selected CSI-RS resources.
 7. The method of claim 6, wherein selecting the subset of the CSI-RS resources is based on measurements of an uplink transmission at one or more of the transmission points.
 8. The method of claim 7, wherein the measured uplink transmission comprises at least one of a Sounding Reference Signal, SRS, a Physical Uplink Control Channel, PUCCH, transmission, and a Physical Uplink Shared Channel, PUSCH, transmission.
 9. The method of claim 6, wherein selecting the subset of the CSI-RS resources is based on measurement data sent by the mobile station to the wireless network.
 10. A method for providing channel-state-information, CSI, feedback in a wireless network that comprises a plurality of geographically separated transmission points sharing a cell identifier, the method comprising: measuring, in a mobile station, signals transmitted from two or more of the transmission points that share the cell identifier, and sending measurements from said measuring to the wireless network; receiving configuration information from the wireless network, said configuration information instructing the mobile station to measure CSI reference symbols, CSI-RS, from at least a first CSI-RS resource used by a first one of the transmission points and a second CSI-RS resource used by a second one of the transmission points, wherein the first and second CSI-RS resources are associated with the shared cell identifier; measuring CSI-RS from the first and second CSI-RS resources; and sending first CSI feedback, corresponding to the first CSI-RS resource, and second CSI feedback, corresponding to the second CSI-RS resource to the wireless network, based on said measuring.
 11. A control unit for use in a wireless network having a wireless network cell that comprises a plurality of geographically separated transmission points sharing a cell identifier, the control unit comprising: a network communication circuit configured to transmit and receive control information to and a plurality of transmission points; and a processing circuit configured to receive channel state information, CSI, feedback from a mobile station, based on measurements of first CSI reference symbols, CSI-RS, transmitted on a first CSI-RS resource from a first one of the transmission points; wherein the processing circuit is further configured to: detect that the mobile station has approached a second one of the transmission points, differing from the first one of the transmission points; reconfigure the mobile station to measure CSI-RS on a second CSI-RS resource transmitted from the second one of the transmission points; and receive CSI feedback from the mobile station, based on measurements of second CSI-RS transmitted on the second CSI-RS resource from the second one of the transmission points.
 12. The control unit of claim 11, wherein the processing circuit is configured to detect that the mobile station has approached a second one of the transmission points by using measurements of an uplink transmission from the mobile station made at the second one of the transmission points and assessing channel strengths based on said measurements.
 13. The control unit of claim 12, wherein the measured uplink transmission comprises at least one of a Sounding Reference Signal, SRS, a Physical Uplink Control Channel, PUCCH, transmission, and a Physical Uplink Shared Channel, PUSCH, transmission.
 14. The control unit of claim 11, wherein the processing circuit is further configured to: configure the mobile station to also measure CSI-RS on a third CSI-RS resource transmitted from a third one of the transmission points; and control the third one of the transmission points to transmit third CSI-RS on the third CSI-RS resource, simultaneously with the transmitting of second CSI-RS from the second one of the transmission points; wherein the CSI feedback received from the mobile station is further based on measurements of the third CSI-RS.
 15. The control unit of claim 11, wherein the processing circuit is further configured to: control all of the transmission points to transmit CSI-RS, wherein at least one distinct CSI-RS resource is used by each of the transmission points; select a subset of the CSI-RS resources for CSI feedback from a mobile station, based on channel strength measurements corresponding to the mobile station and the transmission points; configure the mobile station to provide CSI feedback for the selected CSI-RS resources.
 16. The control unit of claim 15, wherein the processing circuit is configured to select the subset of the CSI-RS resources based on measurements of an uplink transmission at one or more of the transmission points.
 17. The control unit of claim 16, wherein the measured uplink transmission comprises at least one of a Sounding Reference Signal, SRS, a Physical Uplink Control Channel, PUCCH, transmission, and a Physical Uplink Shared Channel, PUSCH, transmission.
 18. The control unit of claim 15, wherein the processing circuit is configured to select the subset of the CSI-RS resources based on measurement data sent by the mobile station to one or more of the transmission points.
 19. The control unit of claim 11, wherein the control unit is part of the primary transmission node.
 20. The control unit of claim 11, wherein the control unit is part of one of the secondary transmission nodes, and wherein the network communication circuit is further configured to transmit and receive control information to and from the primary transmission node.
 21. A mobile station configured to provide channel-state-information, CSI, feedback in a wireless network that comprises a plurality of geographically separated transmission points sharing a cell identifier, the mobile station comprising: a radio circuit configured to receive signals transmitted from the wireless network; and a processing circuit; wherein the processing circuit is configured to: measure signals transmitted from two or more of the transmission points that share the cell identifier and received by the radio circuit; send measurement data from said measuring to the wireless network, using the radio circuit; receive configuration information from the wireless network, via the radio circuit, said configuration information instructing the mobile station to measure CSI reference symbols, CSI-RS, from at least a first CSI-RS resource used by a first one of the transmission points and a second CSI-RS resource used by a second one of the transmission points, wherein the first and second CSI-RS resources are associated with the shared cell identifier; measure CSI-RS from the two or more CSI-RS resources, using the radio circuit; and, using the radio circuit, send first CSI feedback, corresponding to the first CSI-RS resource, and second CSI feedback, corresponding to the second CSI-RS resource, to the wireless network, wherein the first and second CSI feedback are based on the measured first and second CSI-RS, respectively. 